The regulation of binding to stimulatory versus inhibitory receptors by GTP and divalent cations has been comapred in fat cell membranes. The contrast in the regulation of binding to the two classes of receptor lends further support to the notion of discrete GTP regulatory proteins in association with receptors which mediate stimulation or inhibition of adenylate cyclase. Complementation/fusion studies have been carried out between systems replete in dual regulatory components and those which lack either inhibitory receptors or inhibitory GTP regulatory proteins. These studies offer firm evidence of the existence of discrete stimulatory and inhibitory GTP regulatory proteins and provide assay systems for these components, which will permit their purification and characterisation.